Poly(phenylene ether) is a type of plastic known for its excellent water resistance, dimensional stability, and inherent flame retardancy. Properties such as strength, stiffness, chemical resistance, and heat resistance can be tailored by blending it with various other plastics in order to meet the requirements of a wide variety of consumer products, including plumbing fixtures, electrical boxes, automotive parts, and coated wire. Poly(phenylene ether)s are also used as additives to thermoset resins, which are materials that cure to form very hard plastics. The addition of a poly(phenylene ether) can make a cured thermoset resin much less brittle.
For some uses and particularly for use in thermoset compositions, the poly(phenylene ether) can be modified by “capping” the terminal hydroxy groups with a polymerizable group such as a methacrylate ester. The capping reactions are associated with capping agents, capping byproducts, and capping catalysts that can contaminate the resulting capped poly(phenylene ether) and detract from the dielectric properties of a thermoset composition into which the capped poly(phenylene ether) is incorporated. Some methods for separating capping reaction impurities are known, but they have substantial drawbacks.
As described in U.S. Pat. No. 6,897,282 to Freshour et al., precipitating the capped poly(phenylene ether) with an antisolvent reduces the level of capping reaction impurities. However, the capped poly(phenylene ether) yields from precipitation are sometimes poor, particularly for low intrinsic viscosity capped poly(phenylene ether)s, for example capped poly(phenylene ether)s having an intrinsic viscosity of less than or equal to 0.12 deciliters per gram (dL/g) as measured in chloroform at 25° C. Furthermore, the precipitation process creates antisolvent handling and disposal problems, and also produces a capped poly(phenylene ether) with poor solid particle characteristics. The Freshour patent also discloses a method of “washing” (extracting) a capping reaction mixture with water prior to total isolation by devolatilizing extrusion. However, that method requires large volumes of the water wash relative to the volume of the poly(phenylene ether) solution, and the method was ultimately ineffective in that residual levels of impurities in the isolated capped poly(phenylene ether) were high. For example, methacrylic acid levels of 2,668 to 25,003 ppm were observed in the isolated capped poly(phenylene ether).
U.S. Pat. No. 7,589,367 to Carrillo et al. describes a liquid-liquid extraction method using an aqueous caustic soda wash to remove capping reaction impurities. However this method introduces sodium cations into the isolated capped poly(phenylene ether). This is highly undesirable in view of the use of capped poly(phenylene ether) in electrical applications. The presence of metal cations in the capped poly(phenylene ether) can adversely affect its electrical properties, rendering it unsuitable for use in these applications.
U.S. Pat. No. 4,237,265 to Eliassen et al. discloses a cocurrent or countercurrent liquid-liquid extraction in an extractor such that a continuous aqueous phase is maintained while a discrete poly(phenylene ether) solution phase is continuously contacted by the aqueous phase. The aqueous phase is a mixture of water and an alkanol having 1 to 4 carbon atoms. This process requires a relatively large volume of alkanol-water phase relative to the poly(phenylene ether) solution phase, in particular a volume ratio of alkanol-water phase to poly(phenylene ether) solution phase of 1:1 to 10:1. Thus a large amount of alkanol is used, which increases the cost of the process.
There remains a need in the art for an improved liquid-liquid extraction method for the purification of capped poly(phenylene ether). The method should be effective in removing capping agent, capping agent byproduct, and capping catalyst from poly(phenylene ether) reaction mixtures. The method should also use less solvent than the antisolvent precipitation process. In order to further minimize solvent usage, the method should use a low volume ratio of alkanol-water phase to poly(phenylene ether) solution phase. Also, the method should not introduce metal cations into the capped poly(phenylene ether).